OT The maths, the physics, the RF, and the 35mm Elmar

[Nickfed]

I ask this (slightly) off topic here as it is likely to have the most traffic anyway and there appears to be a wide range of members........

It has taken 50 years to work out that I don't understand this.

The focussing on my FED collapsible is as simple as can be. The lens simply rotates on a multi-start thread in its mount. The RF follower bears against the back and swings according to the amount of screw turn applied, thereby in effect indicating the position of the lens relative to the film plane.

A 35mm Elmar looks like it works the same way. But does it? The focussing lever looks the same as the 50mm, Elmar or FED, and moves through about 175 degrees. The focus range is the same, 1m > infinity. The lense moves less to cover the same range. Therefore the pitch of the thread it rotates in cannot be the same. Yet surely the rangefinder follower must move the same amount for 175 degrees rotaton of the focussing lever in order to swing the mirror.

So. Does the 35mm Elmar rotate like a 50mm or is there some tricky internal threading that moves the RF follower the same amount for 175 degrees of rotation but has a different pitch so that the lense moves less far?

Gee I hope I have described that well. .....

The only other RF camera I have had with interchangeable lenses is my Super Graphic and that solves the problem painlessly by having interchangeable cam plates to swing the RF mirror the same amount for differing lenses.

 

[laptoprob]

Welcome Nickfed, to the forum.
The rangefinder cam movement to the camera in a 50mm lens is 1:1. The optical system moves the same as the rangefinder cam. In a telephoto lens the cam moves less and in a wideangle lens the cam moves more to achieve the proper rangefinder in the camera.
The optics are simply very different, require a different helical to move the rangefinder properly.

 

[AndersG]

So. Does the 35mm Elmar rotate like a 50mm or is there some tricky internal threading that moves the RF follower the same amount for 175 degrees of rotation but has a different pitch so that the lense moves less far?

I don't know for sure, but wouldn't a sloped (as opposed to flat) rangefinder cam do the trick?
That way the RF follower could be made to move more or less than the moving part of the lens.

/AndersG

 

[Nickfed]

I have not looked at the back of the Elmar, but did have the Nikkor 3.5cm f3.5 which is a copy of the Elmar. It had a cut out on the back of the lens that coupled with the RF pickup of the camera. This translated the travel of the 3.5cm helical into what the camera expected for a 51.6mm focal length. Look at the back of the Elmar. If it does not use this simple method, then it uses a second helical.

Thanks. I assume the cutout bit you refer to is a ramp on the rotating ring and thus moves the RF follower faster than the lens moves. But I now have a picture of an Elmar 35 and it is very confusing. There is an extension of the mount with a cutout at 4 o'clock. I've no idea about this but it surely isn't part of the focussing. I think the lens barrel is the brassy bit and I fancy this is not parallel with the mount but rises to the ten o'clock area, as AndersG suggests. I'm not sure.

 

[pshinkaw]

The follower (the part that contacts the cam) is the shiny brass ring in the photo. As you focus closer, the ring will move more toward the rear (deeper into the body).

Keep in mind that the Soviet rangefinder coupling design is slightly different from the the other LTM designs. The cam and arm couple the lens through most of its focusing range. Near to almost infinity. However, there is a separate stop (the infinity adjustment screw) that stops the rangefinder movement when the lens reaches (or almost reaches) its infinity position. That stop position on the rangefinder is adjustable.

-Paul

 

[Nickfed]

And treading more familar ground in the FSU.........
With the Jupiter 12 35mm, does the entire lens rotate in its mount while focussing? I only mentioned the Elmar 35mm because I'm pretty certain it does, and the only lens I have ever had that does is the Elmar copy on the FED.

 

[Kin Lau]

The Elmar 35 rotates when focusing, so does the 50/3.5 Elmar but my Summar doesn't. On the FSU side, the I-22 and J8 rotate, but the I-26 & I-61 does not.

The picture of the 35 Elmar above has the lense at the closest focusing distance, 1m. The "cutout" is so that the cam will properly sit against the brass focusing ring. At infinity, the brass ring will rise above the mounting ring on the outside.

From 1m to infinity, the lense itself moves about 2mm but the RF follower move about 3mm.

 

[Nickfed]

I don't get that. Won't the bit that is not cut out foul the cam when the lens is being screwed in?

 

[ChrisN]

I don't get that. Won't the bit that is not cut out foul the cam when the lens is being screwed in?

Just checked this on a Canon with a CV 35/2.5, which has a similar cutout.. It does cause the rangefinder arm to oscillate as the lens is screwed in, and I can see that through the viewfinder. However, once the lens is screwed in, the cutout is lined up, and allows the rangefinder arm (with its little ball-bearing follower) to move further forward, which allows the 0.7m closest focus distance with this particular lens.

Probably different with other lenses/camera. Your milage may vary.

 

[wyk_penguin]

I don't get that. Won't the bit that is not cut out foul the cam when the lens is being screwed in?

He pretty much explained himself. When mounting or dismounting, the lens is set at infinity "the brass ring will rise above the mounting ring" (as viewed from the back.) Thus the cam is riding on the brass ring, being pushed back, and the chrome thread don't get in the way. The dent on the chrome part is there so that when focusing at near objects, i.e. when the cam moves forward, it doesn't get blocked by the threads.

 

[Kin Lau]

I don't get that. Won't the bit that is not cut out foul the cam when the lens is being screwed in?

If you have it set at the close focusing distance, then yes, the RF does move quite a bit, but only for one turn. The Leica's & my Bessa R have a round RF cam and I suspect the Canon's would also, so it would never get fouled or caught. I've never tried mounting this on a FSU cam since they have a wedge cam. You'd probably just have to make sure that you mount it while set at infinity so it doesn't get caught.